1. Technical Field
The present invention relates to a control apparatus for an internal combustion engine in which evaporated fuel is provided to a combustion chamber via a purge passage, a purge control valve, and an intake passage.
2. Background Art
A conventional control apparatus for an internal combustion engine is known, in which evaporated fuel generated in a fuel tank is provided to a combustion chamber through a purge passage with a purge control valve and an intake passage. Providing the evaporated fuel to the combustion chamber is referred to as “evaporated fuel gas purge (or, “evapo-purge” for short)”.
One of such control apparatuses carries out the evaporated fuel gas purge while an air-fuel ratio feedback control is being performed. In the air-fuel ratio feedback control, an air-fuel ratio of a mixture provided to the engine (an air-fuel ratio of the engine) is detected by an air-fuel ratio sensor disposed in an exhaust passage, a feedback correction coefficient for a base (fuel) injection amount is calculated based on the detected air-fuel ratio. An instructed injection amount sent to a fuel-injector is determined through correcting the base injection amount by the feedback correction coefficient. A fuel whose amount corresponds to the instructed injection amount is injected from the injector. Typically, the base injection amount is a feedforward control amount determined based on a load of the engine and a revolution speed of the engine so as to make the air-fuel ratio of the engine become equal to a stoichiometric (theoretical) air-fuel ratio.
For performing the evaporated fuel gas purge, the fuel tank is communicated with the intake passage via the purge passage. A canister is disposed in the purge passage. The purge control valve is disposed downstream of the canister (at the side of the intake passage of the engine) in the purge passage. The evaporated fuel generated in the fuel tank is introduced into the canister through the purge passage, and is adsorbed in the canister tentatively. The evaporated fuel adsorbed in the canister is introduced into the intake passage as the evaporated fuel gas when the purge control valve is opened. In this manner, the evaporated fuel gas purge is carried out (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. Hei 5-202817, FIG. 3).
Meanwhile, the mixture burnt in the combustion chamber includes the fuel injected from the injector and the evaporated fuel introduced via the purge passage, when the evaporated fuel gas purge is being carried out. Therefore, the feedback correction coefficient calculated based on the detected air-fuel ratio includes a correction amount for the evaporated fuel. Accordingly, the feedback correction coefficient excessively decreases the base injection amount, when the evaporated fuel gas purge is stopped. As a result, there may be a case where the air-fuel ratio of the engine becomes excessively large (lean). In view of the foregoing, the control apparatus disclosed in the above Japanese Patent Application performs the following control.
The control apparatus calculates a purge correction coefficient for compensating for a deviation of the air-fuel ratio from the stoichiometric air-fuel ratio due to the evaporated fuel gas purge. Specifically, the control apparatus gradually decreases the purge correction coefficient as an elapsed time from a start timing of the evaporated fuel gas purge increases, based on the perspective that a purge amount of the evaporated fuel gas increases as the elapsed time from the start timing of the evaporated fuel gas purge increases. Further, the control apparatus calculates the feedback correction coefficient based on the detected air-fuel ratio to make the air-fuel ratio of the engine becomes equal to the stoichiometric air-fuel ratio, even when the evaporated fuel gas purge is being carried out (during the purge). The control apparatus corrects the base injection amount based on the purge correction coefficient and the feedback correction coefficient during the purge.
In addition, the control apparatus resets the purge correction coefficient when it stops the evaporated fuel gas purge by completely closing the purge control valve. That is, the control apparatus corrects (sets) the purge correction coefficient to a base value “1”, which neither increase nor decrease a fuel injection amount. At the same time, the control apparatus resets the feedback correction coefficient when it stops the evaporated fuel gas purge, if the feedback correction coefficient is a value which decreases the base injection amount. That is, the control apparatus corrects (sets) the feedback correction coefficient to a base value “1”, which neither increase nor decrease the fuel injection amount.
As a result, it can be avoided that the air-fuel ratio of the engine becomes excessively large (lean) relative to the stoichiometric air-fuel ratio immediately after the evaporated fuel gas purge is stopped, because the feedback correction coefficient is set to be a value which has not been affected by the evaporated fuel gas purge. Consequently, harmful gases, such as NOx, can be reduced.